Precipitation related to synoptic disturbances in Antarctica
Highest accumulation of precipitation in Antarctica is found along the coastal escarpment. This is mainly caused by orographic lifting of relatively warm and moist air in an onshore flow. Large portions of precipitation in the coastal regions fall during episodes of passing fronts of cyclonic systems.
Such a weather situations, formation and horizontal distribution of clouds and precipitation in Dronning Maud Land are investigated using a high-resolution nonhydrostatic weather forecast model, namely the COSMO-Model (COSMO: Consortium of Small Scale Modeling), which was initially developed at the German Weather Service (DWD). This model was applied applied for the first time for Antarctic conditions by our group. Emphasis is placed on the temporal evolution and horizontal distribution of clouds and precipitation
in fine spatial resolution (7 km horizontal mesh size).
For illustration, the case is addressed for an episode with heavy precipitation from February 1999. Figure 1 shows that the simulation captures the observed meteorological situation and the onset of precipitation well (Figure 1).
The horizontal distribution of precipitation (Figure 2) is dominated by topographic effects: The simulations show the general decrease of precipitation amount toward the interior, similar as seen in accumulation climatologies given in text books. But the decrease is not at all monotonous in the high resolution simulation.
Instead, precipitation bands of some 100 km width appear on the plateau. Such mesoscale features can only be resolved by models with mesh sizes of 10 km or less, such as the applied COSMO model.
Figure 1
Figure 1:
Composite time series of accumulated preciptation, valid at a model grid point near Neumayer Station for 2 February 1999 00 UTC to 12 February 1999 12 UTC. Solid, dashed and dash-dotted lines refer to different skipped spin-up periods. Thick lines mark the reference run, thin lines a model run with a modified surface condition (not addressed here). Asterisks mark whether the
synop data report precipitation (value > 0) or not (value 0), but do not
indicate the amount of precipitation.
Figure 2
Figure 2:
Horizontal distribution of simulated preciptitation sums (in mm water equivalent) valid from model runs as used for Figure 1. The first 12 hours of each simulation are skipped as spin-up time.
Spatial and temporal variations of precipitation modify the signatures, due to e.g. temperature, stored in the snow and hence, are relevant for the interpretation of ice cores.
Another important point is the 'spin-up time' for simulated precipitation. The spin-up time is the early period of a model run, when the precipitation results are still affected by deficits in the initialization of hydrological variables. The results from this spin-up time have to be discarded from
physical interpretation. Our experience from the model studies for Antarctica is that the spin-up time for precipitation is 6 to 12 hours, see Figure 1.
For more details see:
Wacker, U., Ries, H., Schaettler, U. (2009): Precipitation for Dronning Maud Land using the COSMO Model. Antarctic Science (21), 643 - 662.
Parts of the research were supported by the Priority Research Programm 'Antarctic research' funded by the Deutsche Forschungsgemeinschaft, grant WA1334/4.